Protein tyrosine kinases are enzymes which show the property of catalysing the transfer of phosphate groups from donor molecules (ATP) to the hydroxyl groups of tyrosine residues in polypeptides. Known tyrosine kinases can be classified into two broad groups. Transmembrane tyrosine kinases traverse cellular membranes so that they possess extracellular and intracellular domains. Cytoplasmic tyrosine kinases are located only intracellularly. A general feature of transmembrane (also referred to as receptor) tyrosine kinases is that they possess extracellular ligand-binding domains, hydrophobic transmembrane sequences, and intracellular portions which include the tyrosine kinase domains (for a review see Ullrich & Schlessinger Cell, 61, 203-212 (1990)).
Tyrosine kinases can induce cell proliferation, cell transformation and regulate developmental events (see reviews by Hanks et al, Science, 241, 42-75 (1988) and Cantley et al, Cell 64, 281-302 (1991)). In general, where functional assays have been available (e.g. mitogenesis or transformation), it has been shown that the biological functions of tyrosine kinases are usually dependent on intact enzyme activity and that through autophosphorylation and the phosphorylation of other proteins, they alter the subcellular localisation and activities of various components of the intracellular signalling pathways.
Analysis of the oncogenes of many acutely transforming animal retroviruses has revealed that their products frequently manifest tyrosine kinase activity, as do the products of their cellular proto-oncogene counterparts. Other genes encoding tyrosine kinases have been found to be altered by DNA rearrangements in cancer cells, the result being a presumed acquisition of cellular transforming activity (for example c-abl, c-met). Some cellular proto-oncogenes encoding tyrosine kinases have been cloned independently by virtue of the fact that they encode growth factor receptors, for example the epidermal growth factor receptor. On the other hand, the genes for other growth factor receptors which have tyrosine kinase activities, such as those for platelet-derived growth factor and insulin-like growth factors, have been well characterised, but have never been found to be transduced by retroviruses.
The c-erbB-2/HER2/c-neu gene encodes a transmembrane receptor-like tyrosine kinase which is structurally very similar to the EGF/TGF alpha receptor. The c-erbB-2 gene has been found to be overexpressed in 20 to 30% of human breast tumours, often in association with gene amplification, and this phenotype is now generally accepted as predictive of poor disease free and overall survival (for reviews see Sunderland & McGuire in Regulatory Mechanisms in Breast Cancer, Lippman & Dickson (Eds.), Kluwer Academic Publishers, Boston, pages 3 to 22 (1991) and Gusterson et al., J. Clin. Oncol., 10, 1049-1056 (1992)). Overexpression of the gene in fibroblasts induces transformation (di Fiore et al, Science, 237, 178-182 (1987) and Hudziak et al, Proc. Natl. Acad. Sci. (USA), 84, 7159-7162 (1987)), and ligands which bind to the c-erbB-2 gene product (human and rodent) and activate its tyrosine kinase activity have been identified (Lupu et al, Science, 249, 1552-1555 (1990), Dobashi et al, Proc. Natl. Acad, Sci. (USA), 88, 8582-8586 (1991), Wen et al, Cell, 69, 559-572 (1992) and Holmes et al, Science 256, 1205-1210 (1992)). Some of these ligands increase the rate of proliferation of cells expressing the c-erbB-2 protein. The strong implication of these and other findings is that overexpression of this tyrosine kinase in some breast tumour cells is one of the important steps in their progression towards tumourigenicity, and therefore that c-erbB-2 can function as an oncogene.
A variety of other receptor tyrosine kinases, including those for the IGFs and the FGFs, are expressed in breast tumours (Stewart et al, J. Biol. Chem., 265 21172-21178 (1990) and Wellstein & Lippman in Molecular Foundations of Oncology, Broder (Ed.), Williams and Wilkins, Baltimore, pages 403-418 (1991)), and the EGF/TGF alpha receptor is overexpressed in some cases (for example Horak et al, Oncogene, 6, 2277-2284 (1991)). As with c-erbB-2, overexpression of EGFr has been correlated with poor prognosis. Since the ligands of some of these receptors can be detected in breast tumour samples, it may be that breast tumour development is also regulated by these molecules (for a review see van de Vijver & Nusse, Biochim, Biophys. Acta, 1072, 35-50 (1991)).
The features which are ideally required in a molecular target for tumour therapies include preferential expression in the tumours, a role in the development of the tumours (rather than merely being a marker), and a knowledge of the mechanism of its action as a starting point for the rational design of activity modulators. It appears that all of these three criteria are satisfied, at least to some extent, in the case of c-erbB-2 and breast cancer and a considerable research effort is currently being directed to this molecule.
It has recently been found that the alkaloid K252a, at appropriate concentrations, selectively inhibits the tyrosine kinase and biological activities of the trk-class of neurotrophin receptors, but has no effect on the activities of v-src, v-fms or the receptors for EGF and PDGrF, and also does not affect general cell viability or proliferation (Tapley et al, Oncogene, 7, 371-381 (1992)). Members of another class of molecules, the tyrphostins, may be selective inhibitors of the EGFr/c-erbB-2 class of tyrosine kinases (Gazit et al, J. Med. Chem., 34, 1896-1907 (1991)). An antibody which inhibits the proliferation of human tumour cells by binding to the c-rbB-2 protein is currently uder evaluation as a potential cancer therapy (Carter et al, Proc. Natl. Acad. Sci. (USA), 89, 4285-4289 (1992))
If the transformation of a significant minority of breast tumour cells is due, at least in part, to the disregulated activity of the c-erbB-2 tyrosine kinase, then it seems likely that other breast tumours owe aspects of their transformed phenotype to the activity of other tyrosine kinases. The identification of further tumour associated tyrosine kinases would allow the development of novel diagnostic and therapeutic approaches to cancer and in particular breast tumours.